1. Field of the Invention
The invention relates to an inexpensive process for the continuous catalytic hydrogenation of sugars, such as D-xylose, .alpha.-D-glucose, 4-O-.beta.-D-galactopyranosyl-.alpha.-D-glucopyranose or 4-O-.alpha.-D-glucopyranosyl-.alpha.-D-glucopyranose, with hydrogen to give the corresponding sugar alcohols, such as D-xylitol, D-sorbitol, 4-O-.beta.-D-galactopyranosyl-.alpha.-D-sorbitol or 4-O-.alpha.-D-glucopyranosyl-.alpha.-D-sorbitol.
The course of the reaction is given by the following reaction schemes: ##STR1##
2. Description of the Related Art
To prepare xylitol (DE-A 19 35 934) or sorbitol (DE-C 544 666; DE-C 554 074), hitherto, use has principally been made of a batch process in which a pulverulent nickel catalyst is used in a suspension process. To prepare lactitol, which has not hitherto been detected in nature, EP-A 39 981 also discloses a batch process in which likewise a pulverulent nickel catalyst is used in a suspension process. A process of this type is also proposed in U.S. Pat. No. 3,741,776 for the preparation of maltitol.
Batch suspension processes have the disadvantage that their capacity, relative to the reaction volume, is very low and there is thus a need for high-volume expensive reaction apparatuses and storage tanks. The energy consumption is uneconomic and the labour requirements are relatively high. Continuous powder catalyst processes which operate with two or more batch hydrogenation reactors connected in cascade only partially avoid said disadvantages. There is still the laborious task of specifically metering the pulverulent catalyst, activating it, circulating it by pumping and quantitatively filtering it off from the reaction product. The catalyst pumps are subject to high mechanical stress. Quantitative removal of the pulverulent catalyst is complex (alternating coarse and fine filtration apparatuses). In addition, there is a high risk that the catalyst relatively rapidly loses its activity due to the additional operations (high catalyst consumption). It is therefore desirable to make the reaction proceed on fixed-bed cataysts, which should have a high specific activity which as far as possible should not decay even over a prolonged period of several years, since frequent changes of catalyst in fixed-bed reactions would also be expensive. In the case of fixed-bed catalysts, also, it has been conventional hitherto to connect a plurality of reactors one after the other, which gives a plurality of series-connected reaction zones (German Offenlegungsschrift 3 214 432).
Use is predominantly made here of nickel catalysts on oxidic support material (SiO.sub.2 /Al.sub.2 O.sub.3) having extremely high active surface areas of 140 to 180 m.sup.2 /g, so that the catalysts in the initial phase are frequently so active that they must be stabilized by additional chemical treatment methods, for example by treatment with oxygen gas to form monomolecular oxygen layers on the catalyst surface (German Offenlegungsschrift 3 110 493). However, the deactivating stabilization of the catalyst then requires reaction temperatures in the hydrogenation which are so high (130 to 180.degree. C.) that uncontrollable side reactions become possible, such as discoloration by caramelization and hydrogenating cracking (hydrogenolysis) of the sugar alcohols to the formation of methanol and even methane. In addition, in this mode of reaction, relatively large amounts of heavy metals constantly pass into solution in ionic or colloidal form which, on the one hand, requires subsequent activated carbon treatment of the hydrogenated product and, on the other hand, requires deionization by ion exchangers.
Since most hydrogenation processes operate with sugar solutions set to pHs of 7 to 13, the acidic starting solutions must be admixed with alkali metals or alkaline earth metals, which likewise must be laboriously removed again (German Offenlegungsschrift 3 110 493; German Offenlegungsschrift 3 214 432). In addition, a marked epimerization would be expected under the hydrogenation conditions, so that, for example, D-xylose would also produce, in addition to xylitol, lyxitol (or arabinitol and ribitol). .alpha.-D-Glucose, in addition to sorbitol, would also be expected to produce mannitol. In addition, the effect of cleaving the carbon chain of sugars during the catalytic hydrogenation by Raney nickel is known; German Offenlegungsschrift 2 756 270 describes the effect on a sugar mixture, as originates from the self-condensation of formaldehyde, a marked shift from higher C chain numbers to lower C chain numbers being observed in the illustrative examples given there.
EP-A 423 525 (a counterpart of U.S. Pat. No. 5,162,517) discloses a process for the continuous hydrogenation of sugars to the corresponding epimer-free sugar alcohols on support-free solid bodies of elements of the iron subgroup of subgroup VIII of the Periodic Table of the Elements, these support-free shaped bodies preferably having been prepared by pressing and/or bonding metal powders. In this case it was found that the sugars are not only substantially converted, but chiefly only one sugar alcohol in each case is produced with substantial avoidance of epimerization and C-chain cleavage and with avoidance of the formation of higher-molecular components by condensation reaction with ether formation.
EP-A 694 515 discloses a more inexpensive process for the preparation of sugar alcohols selected from the group consisting of xylitol, sorbitol, 4-O-.beta.-D-galactopyranosyl-.alpha.-D-sorbitol (lactitol) and 4-O-.alpha.-D-glucopyranosyl-.alpha.-D-sorbitol (maltitol) by catalytic hydrogenation of the corresponding sugars D-xylose, .alpha.-D-glucose, 4-O-.beta.-D-galactopyranosyl-.alpha.-D-glucopyranose and 4-O-.alpha.-D-glucopyranosyl-.alpha.-D-glucopyranose, repectively, on support-free shaped bodies made of elements of the iron subgroup of subgroup VIII. of the Periodic Table of the Elements including activating elements of subgroup VI. However, it continues to be desirable to increase the low conversion rates (g of sugar /1 of catalyst.times.h) and to further reduce the catalyst costs. Furthermore, the aim is still to carry out a process in as high a concentration as possible of the substance to be hydrogenated in the solvent and at a temperature as low as possible in order to decrease further the energy costs also.